1. Field of the Disclosure
The present invention relates generally to switching power converters and, more specifically, the present invention relates to a technique to regulate an output of a switching power converter.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device, typically referred to as a power converter, is required to transform the high-voltage AC power to low-voltage DC power. The low-voltage DC power may be provided by the power converter directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. Typically, the battery is charged with a battery charger that includes a power converter that meets constant current and constant voltage requirements required by the battery. In operation, a power converter may use a controller to regulate output power delivered to an electrical device, such as a battery, that may be generally referred to as a load. More specifically, the controller may be coupled to a sensor that provides feedback information of the output of the power converter in order to regulated power delivered to the load. The controller regulates power to the load by controlling a power switch to turn on and off in response to the feedback information from the sensor to transfer energy pulses to the output from a source of input power such as a power line.
One particular type of power converter that may be used is a flyback power converter. In a flyback power converter, an energy transfer element separates the input of the power converter from the output of the power converter. The energy transfer element typically provides isolation that prevents DC current from flowing between the input and the output. In applications where isolation is not required, the input and the output may share a common return terminal that allows DC current to flow between the input and the output. A switching power converter produces an output by periodically switching a power switch for one or more switching cycles. An on/off control technique that regulates an output of a switching power converter is one that enables or disables conduction of the power switch within a particular switching cycle. When enabled, the power switch may conduct current within a switching cycle. When disabled, there is no conduction for the entire duration of the switching cycle.
A current limited on/off control is one that terminates the conduction of the power switch within an enabled switching cycle when the current in the switch reaches a current limit. When conduction is enabled, the power switch may conduct within the switching cycle until the current in the power switch reaches a current limit or until the conduction time reaches a maximum value.
An apparatus to implement an on/off control technique in a power converter is generally referred to as an on/off controller. A typical on/off controller decides whether to enable or to disable the conduction of the power switch for each switching cycle by sensing the output of the power converter at the end of the previous switching cycle. The controller may also set the current limit of the switch to control the energy delivered to the output in each switching cycle. By controlling the energy delivered to the output in a given cycle, the output may be influenced, thereby having an effect on whether conduction of the power switch is enabled or disabled in subsequent switching cycles. On/Off control has advantages over alternatives in many applications that demand special combinations of features such as simplicity, low cost, fast transient response, and low power consumption at no load.